(1) Field of the Invention
The invention relates to the general field of ion implantation, more particularly to the measurement of ion dosages, and specifically to implanted nitrogen and silicon.
(2) Description of the Prior Art
Ion implantation is widely used as a method for introducing foreign material into semiconductors during the manufacture of integrated circuits. Most commonly, the foreign atoms that are inserted into the semiconductor are dopants such as (for silicon) arsenic, phosphorus, and boron.
In recent years, as the critical dimensions of integrated circuits (ICs) continue to shrink, a need has arisen for the implantation of additional silicon atoms into certain parts of the ICs. This is a side effect of the salicide (Self aligned silicide) process wherein a conductive layer of a material such as titanium silicide is selectively formed only on silicon surfaces. The process depends on the fact that wherever the (titanium) metal is in direct contact with silicon it will, on heating, react to form the silicide, metal elsewhere remaining unchanged. Thus, following the heat treatment, all unreacted metal gets removed and only the original silicon surface retains a conductive overcoat.
Since the silicidation reaction consumes a certain amount of silicon at the metal-silicon interface, there arises the possibility that very thin layers will be fully consumed and shorted through. To prevent this, additional silicon needs to be added to the areas that will be affected (in practice, all source, drain, and gate electrode areas) by the silicidation. Ion implantation is the preferred technique for accomplishing this because it is simple, cheap, and uniformity is easy to control.
FIG. 1 is an illustration of a typical ion implantation apparatus of the type used in the IC industry. See, for example, Chu (U.S. Pat. No. 5,604,350 Febuary 1997). The ion species that is to be implanted is introduced into ion source chamber 1 as an elemental gas or as part of a gaseous compound. Silicon ions are most commonly generated from silicon tetrafluoride which dissociates inside chamber 1 into fluorine and silicon ions.
All ions that are formed in chamber 1 are extracted through gate 2 so that they pass through analyzing magnet 3 which allows only ions having the intended atomic mass to pass into acceleration tube 4 where the selected ions acquire the high voltage needed for effective ion implantation. After the ion beam has been focussed it is electrically diverted at 6 so that any neutral species in the beam get removed at 7. Finally, the focussed ion beam is directed at silicon wafer 8. Since the focus area of the beam is small, scanning plates 9 are also provided to allow the ion beam to cover larger areas. Also included with the wafer is means for measuring the beam current of the ion beam so that total accumulated ion dosage may be computed.
In the more common applications of ion implantation, the species selected by analyzer 3 are unique so the ion implantation process does not introduce any contaminants. In the case of silicon ions there is a problem with nitrogen as a contaminant because nitrogen, which has an atomic weight of 14, emerges from chamber 1 primarily as ionized nitrogen molecules (N.sub.2.sup.+) which have a molecular weight of 28--the same value as the atomic weight of silicon.
Unfortunately, nitrogen has the potential to be a major contaminant in ion implantation systems since they must constantly be exposed to the air during loading and unloading. Time is money in production systems so there is always some uncertainty as to whether or not sufficient time has been allowed for all nitrogen to be removed from the ionization chamber before implantation begins.